My dad always said there are two kinds of married people: those who are committed, and those who probably ought to be.

He was kidding, of course. “Commitment” is sort of the definition of marriage — or at least it used to be, before people began looking at their wedding vows the way college football coaches look at their contracts.

I recently heard a divorce attorney say on the radio that “no one goes into a marriage thinking it will end.” Maybe not — but a lot of people go into marriage thinking it COULD end, if “things don’t work out.”

And therein lies the problem: If you start with the assumption that marriage is impermanent, it’s very likely to be.

Why do so many people start with that assumption? Because they look around and see that it’s so. Half the people they know are divorced — including, in many cases, their own parents. So what’s the big deal?

I’ll leave you to decide if you think it’s a big deal. But let’s assume you do, since you’re reading this column. If you truly want to have a long and happy marriage, you must put out of your mind the idea that you can always walk away.

You can’t. That’s what “commitment” means.

A good friend of mine was once asked by his wife that immortal question, “Why do you love me?” He replied, “Because I choose to.”

At first, she wasn’t exactly thrilled with that answer. She wanted him to say something more romantic, about how beautiful and wonderful she is. But eventually she came to see that he was exactly right. Marriage is ultimately about choosing each other — not just on the day you get engaged or the day you say your vows, but every single day for the rest of your lives.

Even days you don’t feel like it. Especially days you don’t feel like it.

Note that I used the word “love” in my anecdote about commitment. That’s because, when it comes to marriage, love and commitment are essentially the same thing.

The classic mistake young people (and sometimes older people) make when they first get married is assuming the feelings of strong affection and sexual desire they have for each other on that day — in other words, what we normally call “love” — will remain unchanged throughout their lives.

People who have been married a long time, however, know that such feelings tend to ebb and flow and thus form a poor basis for a lasting marriage. They also understand that you’re more likely to still feel those emotions, years into the marriage, if you continue to choose each other even on days when you don’t feel them.

In other words, most people believe that commitment follows love. But one of the secrets to a long and happy marriage is understanding that, in reality, true love follows undying commitment.

Rob Jenkins is a local freelance writer and the author of “Family Man: The Art of Surviving Domestic Tranquility,” available at Books for Less and on Amazon. E-mail Rob at rjenkinsgdp@yahoo.com or follow him on Twitter @FamilyManRob.